The maternal response in terms of changes in weight, plasma volume, total body water, intravascular protein mass and urinary oestriol excretion to a singleton pregnancy has been shown to be related to the size of the baby produced. Twin pregnancies have been studied to assess the maternal response to the double load, to see whether a much greater response is required compared to a singleton pregnancy. It is hoped that this study will eventually show not only what can be considered to be a physiological response to a twin pregnancy, but will show how the maternal organism sometimes fails to respond normally to the double load.

Observations were made at 30 to 32 weeks and again between 36 and 39 weeks of pregnancy in 14 primigravid twin pregnancies and 17 multigravid twin pregnancies, as well as in singleton primigravidae. For the purposes of comparison the singleton pregnancies have been divided up by the amount of weight gain during pregnancy. High weight gain was defined as 600 g/week or more during the 20th to 30th weeks. Normal weight gain was 360-540 g/week and low weight gain 320 g/week or less. The observations made were plasma volume using dye T. 1842 (Evan's Blue); total body water using deuterium oxide and measuring in an infrared spectrophotometer; the intravascular protein mass; the red cell mass; and the urinary oestriol excretion.